1. Field of the Invention
The present invention relates to a silver halide emulsion and in particular to a photographic silver halide emulsion in which the formation of photographic fog caused by a sensitizing dye is inhibited and the reduction in spectral sensitivity with the lapse of time is prevented.
2. Description of the Prior Art
It is well known that optical sensitization, i.e., a technique for further extending the light-sensitive wavelength range of a photographic silver halide emulsion to longer a wavelength side by adding a certain sensitizing dye to the emulsion, can be employed as a technique for producing a photographic light-sensitive emulsion.
The intensity of the spectral sensitization is influenced by the chemical structure of the sensitizing dye, the properties of the emulsion (such as, for example, the silver halide composition, the silver halide crystal habit and the silver halide crystal system, the silver ion concentration and the hydrogen ion concentration), and the like. Further, the spectral sensitivity is also affected by photographic additives also present in the emulsion, such as stabilizers, anti-foggants, coating aids, precipitating agents, color couplers, and the like.
It is also well known that the addition of a sensitizing dye to a silver halide emulsion results in photographically adverse side effects along with spectral sensitization. Of these adverse effects, the formation of fog caused by the addition of a sensitizing dye (hereinafter, this type of fog will be referred to as "dye fog") and the reduction in spectral sensitivity with the lapse of time greatly hinder the production of a photographic light-sensitive material having sufficient optical sensitivity, reduced fogging, and good storability. In particular, in the case of photographic papers, even the slightest increase in dye fog undesirably reduces the commercial value of the photographic papers to a great extent. When a sensitizing dye is used together with a conventionally employed anti-foggant or stabilizer for improving these drawbacks, the spectral sensitivity is remarkably reduced in some cases and degradation of spectral sensitivity with the lapse of time may be rather increased. Thus, for obtaining effective results, the above-mentioned additives must be selected strictly in accordance with the sensitizing dye to be employed.
On the other hand, when a dinuclear merocyanine dye is used as a sensitizing dye in a color photographic material, the dye tends to diffuse into other layers, whereby the diffused dye undesirably sensitizes the layer into which it has diffused (hereinafter, this type of sensitization will be referred to as "diffusion sensitization"). For example, on diffusing into a green-sensitive layer and/or a blue-sensitive layer, a merodicarbocyanine dye employed in a red-sensitive layer may panchromatically sensitize these layers, resulting in a production of color staining in the colored images, a reduction of the green sensitivity in the green-sensitive layer, or a reduction of the blue sensitivity in the blue-sensitive layer; on diffusing into a blue-sensitive layer and/or a red-sensitive layer, a merocarbocyanine dye employed in a green-sensitive layer may orthochromatically sensitize this layer, resulting in a color mixing in the colored images, a reduction of the blue sensitivity in the blue sensitive layer, or a reduction in the red sensitivity in the red-sensitive layer; or on diffusing into a green sensitive layer and/or a red-sensitive layer, a simple merocyanine dye may cause a reduction in the green sensitivity in the green-sensitive layer, or a reduction in the red sensitivity in the red-sensitive layer. Diffusion sensitization is a phenomenon that is markedly accelerated when a color light-sensitive material is stored under high humidity conditions. Therefore, in cases where a dinuclear merocyanine dye exhibiting high sensitizing effect is employed, prevention of diffusion sensitization is of practical importance.